Method and apparatus for sensing improvement using pressure data

ABSTRACT

A method and apparatus for sensing improvement using pressure data. The method and apparatus may be used in an implantable medical device to confirm that an EGM event signifies a true mechanical cardiac activity and not just electrical oversensing. The mechanical activity may be used to create a mechanical marker channel in the implantable medical device.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/741,942, filed Apr. 30, 2007 entitled “METHOD AND APPARATUS FORSENSING IMPROVEMENT USING PRESSURE DATA”, herein incorporated byreference in its entirety.

BACKGROUND

Implantable medical devices (IMDs), such as cardiac pacemakers anddefibrillators, are useful for management of a variety of cardiacconditions such as congestive heart failure, conduction defects andarrhythmias. Since IMDs typically deliver therapies based on sensedelectrical cardiac activity, the ability of IMDs to accurately detectand interpret cardiac electrical signals is essential to the delivery ofproper therapies.

Implantable medical devices typically sense cardiac electrical activitythrough electrodes implanted in or around the heart and/or otherlocations within the patient's body, which produce cardiac electrograms(EGMs). The quality of the data provided by the electrodes affects theability of the IMD to correctly interpret the cardiac activity. Theelectrical cardiac signals received by the IMD may be negativelyaffected by factors such as pathological changes in the heart'sintrinsic activity, lead maturation effects such as changes in thepositioning of implanted leads, or changes in the conductive propertiesof the heart muscle in the region surrounding the leads, such as mightresult from myocardial infarction and fibrotic tissue growth around thelead. In addition, certain non-cardiac signals, such as electromagneticnoise, myopotentials, and the like, must be distinguished by the IMDfrom true cardiac electrical activity.

The ability of an IMD to sense cardiac signals is typically controllableby means of circuitry for adjusting the sensitivity threshold of thepacemaker's sense amplifier, such that electrical signals resulting fromdepolarization of the cardiac muscle must exceed this threshold in orderfor the cardiac event to be recognized. The sense amplifier circuitry ofthe IMD must be sensitive enough to ensure detection of cardiac signals,which are typically of relatively low magnitude, especially in the caseof atrial sensing. However, the sense amplifier must not be so sensitivethat certain non-cardiac signals, such as electromagnetic noise,myopotentials, and the like, cause the IMD to erroneously sense acardiac signal which did not actually occur. For example, in the case ofpacemakers, if the sense amplifier circuitry is not sensitive enough(undersensing), the pacemaker could lose synchronization with thenatural cardiac rhythm or deliver pacing stimuli at inappropriate times.However, if the sense amplifier circuitry is set too low (oversensing),the pacemaker could erroneously sense a cardiac signal which did notoccur. Similarly, defibrillators which are oversensing could detect anarrhythmia and deliver an inappropriate spurious shock. Thus, whilesensitivity adjustments help to refine the ability of an IMD to detectelectrical signal, undersensing, oversensing and poor signal qualitycreate a risk that the IMD may incorrectly interpret an electricalsignal. An improved system or method that provides the appropriate levelof sensing is therefore desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are intended for use in conjunction with the explanationsin the following detailed description. Embodiments of the presentinvention will hereinafter be described in conjunction with the appendeddrawings, wherein like numerals denote like elements.

FIG. 1 is a schematic diagram depicting a multi-channel, atrial andbi-ventricular, monitoring/pacing implantable medical device (IMD) inwhich embodiments of the invention may be implemented;

FIG. 2 is a simplified block diagram of an embodiment of IMD circuitryand associated leads that may be employed in the system of FIG. 1 toenable selective therapy delivery and monitoring in one or more heartchamber;

FIG. 3 is a simplified block diagram of a single monitoring and pacingchannel for acquiring pressure, impedance and cardiac EGM signalsemployed in monitoring cardiac function and/or delivering therapy,including pacing therapy, in accordance with embodiments of theinvention;

FIG. 4 is simultaneous recording over time of an EGM, right ventricularintracardiac pressure, and right ventricular dP/dt, with a markerchannel M set at a threshold right ventricular dP/dt of 100 mmHg/s;

FIG. 5 is a simultaneous reading over time of an EGM, right ventricularintracardiac pressure, and right ventricular dP/dt, simulating T-waveoversensing on the EGM recording;

FIG. 6 is a simultaneous reading over time of an EGM, right ventricularpressure, and right ventricular dP/dt, demonstrating an oversensedT-wave followed by a right ventricular dP/dt measurement window;

FIG. 7 is a simultaneous reading over time of RR intervals, rightventricular dP/dt, RV pulse pressure, and the average pulse pressure ofbeats for which the right ventricular dP/dt exceeds a threshold of 100mmHg/s;

FIG. 8 is a flow chart demonstrating a method of differentiatingR-waves, with and without mechanical sensed events, and T-waves usingpressure data;

FIG. 9 is a simultaneous reading over time of mechanical and electricaldata during an episode of induced ventricular fibrillation; and

FIG. 10 is a simultaneous reading over time of right ventricularpressure and right ventricular dP/dt.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description providespractical illustrations for implementing exemplary embodiments of thepresent invention.

Implantable medical devices (IMDs) may be used to monitor and delivertherapy to a patient's heart. IMDs typically sense a patient's cardiacelectrogram, interpret the electrogram to represent a cardiac rhythm,and deliver therapy based on that interpretation. Accurate electricalsensing and data interpretation are therefore essential to the deliveryof appropriate therapy such as IMDs. Embodiments of this inventionemploy intracardiac pressure data for monitoring cardiac activity. Suchpressure data may be used alone for patient monitoring or in conjunctionwith EGM, such as to confirm the accurate interpretation of EGM data.Certain embodiments of the invention may include, or may be adapted foruse in, diagnostic monitoring equipment, external medical devicesystems, and implantable medical devices (IMDs), including implantablehemodynamic monitors (IHMs), implantable cardioverter-defibrillators(ICDs), cardiac pacemakers, cardiac resynchronization therapy (CRT)pacing devices, drug delivery devices, or combinations of such devices.

FIG. 1 is a schematic representation of an implantable medical device(IMD) 14 that may be used in accordance with certain embodiments of theinvention. The IMD 14 may be any device that is capable of measuringhemodynamic parameters (e.g., blood pressure signals) from within aventricle of a patient's heart, and which may further be capable ofmeasuring other signals, such as the patient's electrogram (EGM).

In FIG. 1, heart 10 shows the right atrium (RA), left atrium (LA), rightventricle (RV), left ventricle (LV), and the coronary sinus (CS)extending from the opening in the right atrium laterally around theatria to form the great vein.

FIG. 1 depicts IMD 14 in relation to heart 10. In certain embodiments,IMD 14 may be an implantable, multi-channel cardiac pacemaker that maybe used for restoring AV synchronous contractions of the atrial andventricular chambers and simultaneous or sequential pacing of the rightand left ventricles. Three endocardial leads 16, 32 and 52 connect theIMD 14 with the RA, the RV and the LV, respectively. Each lead has atleast one electrical conductor and pace/sense electrode, and a canelectrode 20 may be formed as part of the outer surface of the housingof the IMD 14. The pace/sense electrodes and can electrode 20 may beselectively employed to provide a number of unipolar and bipolarpace/sense electrode combinations for pacing and sensing functions. Thedepicted positions in or about the right and left heart chambers aremerely exemplary. Moreover other leads and pace/sense electrodes may beused instead of the depicted leads and pace/sense electrodes.

It should be noted that the IMD 14 may also be an implantablecardioverter defibrillator (ICD), a cardiac resynchronization therapy(CRT) device, an implantable hemodynamic monitor (IHM), or any othersuch device or combination of devices, according to various embodimentsof the invention.

Typically, in pacing systems of the type illustrated in FIG. 1, theelectrodes designated above as “pace/sense” electrodes are used for bothpacing and sensing functions. In accordance with one aspect of thepresent invention, these “pace/sense” electrodes can be selected to beused exclusively as pace or sense electrodes or to be used in common aspace/sense electrodes in programmed combinations for sensing cardiacsignals and delivering pace pulses along pacing and sensing vectors.

In addition, some or all of the leads shown in FIG. 1 could carry one ormore pressure sensors for measuring systolic and diastolic pressures, aswell as incorporating electrodes which are spaced apart which functionas impedance sensing leads for deriving volumetric measurements of thepatient's torso and expansion and contraction of the RA, LA, RV and LV.

The leads and circuitry described above can be employed to record EGMsignals, blood pressure signals, and impedance values over certain timeintervals. The recorded data may be periodically telemetered out to aprogrammer operated by a physician or other healthcare worker in anuplink telemetry transmission during a telemetry session, for example.

FIG. 2 depicts a system architecture of an exemplary multi-chambermonitor/sensor 100 implanted into a patient's body 11 that providesdelivery of a therapy and/or physiologic input signal processing. Thetypical multi-chamber monitor/sensor 100 has a system architecture thatis constructed about a microcomputer-based control and timing system 102which varies in sophistication and complexity depending upon the typeand functional features incorporated therein. The functions ofmicrocomputer-based multi-chamber monitor/sensor control and timingsystem 102 are controlled by firmware and programmed software algorithmsstored in RAM and ROM including PROM and EEPROM and are carried outusing a CPU or ALU of a typical microprocessor core architecture.

The therapy delivery system 106 can be configured to include circuitryfor delivering cardioversion/defibrillation shocks and/or cardiac pacingpulses delivered to the heart or cardiomyostimulation to a skeletalmuscle wrapped about the heart. Alternately, the therapy delivery system106 can be configured as a drug pump for delivering drugs into the heartto alleviate heart failure or to operate an implantable heart assistdevice or pump implanted in patients awaiting a heart transplantoperation.

The input signal processing circuit 108 includes at least onephysiologic sensor signal processing channel for sensing and processinga sensor derived signal from a physiologic sensor located in relation toa heart chamber or elsewhere in the body. Examples illustrated in FIG. 2include pressure and volume sensors, but could include other physiologicor hemodynamic sensors.

FIG. 3 schematically illustrates one pacing, sensing and parametermeasuring channel in relation to one heart chamber. A pair of pace/senseelectrodes 140, 142, a pressure sensor 160, and a plurality, e.g., four,impedance measuring electrodes 170, 172, 174, 176 are located inoperative relation to the heart 10.

The pair of pace/sense electrodes 140, 142 are located in operativerelation to the heart 10 and coupled through lead conductors 144 and146, respectively, to the inputs of a sense amplifier 148 located withinthe input signal processing circuit 108. The sense amplifier 148 isselectively enabled by the presence of a sense enable signal that isprovided by control and timing system 102. The sense amplifier 148 isenabled during prescribed times when pacing is either enabled or notenabled in a manner known in the pacing art. The blanking signal isprovided by control and timing system 102 upon delivery of a pacing orPESP pulse or pulse train to disconnect the sense amplifier inputs fromthe lead conductors 144 and 146 for a short blanking period in a mannerwell known in the art. The sense amplifier provides a sense event signalsignifying the contraction of the heart chamber commencing a heart cyclebased upon characteristics of the EGM. The control and timing systemresponds to non-refractory sense events by restarting an escape interval(EI) timer timing out the EI for the heart chamber, in a manner wellknown in the pacing art.

The pressure sensor 160 is coupled to a pressure sensor power supply andsignal processor 162 within the input signal processing circuit 108through a set of lead conductors 164. Lead conductors 164 convey powerto the pressure sensor 160, and convey sampled blood pressure signalsfrom the pressure sensor 160 to the pressure sensor power supply andsignal processor 162. The pressure sensor power supply and signalprocessor 162 samples the blood pressure impinging upon a transducersurface of the sensor 160 located within the heart chamber when enabledby a pressure sense enable signal from the control and timing system102. Absolute pressure (P), developed pressure (DP) and pressure rate ofchange (dP/dt) sample values can be developed by the pressure sensorpower supply and signal processor 162 or by the control and timingsystem 102 for storage and processing.

A variety of hemodynamic parameters may be recorded, for example,including right ventricular (RV) systolic and diastolic pressures (RVSPand RVDP), estimated pulmonary artery diastolic pressure (ePAD),pressure changes with respect to time (dP/dt), heart rate, activity, andtemperature. Some parameters may be derived from others, rather thanbeing directly measured. For example, the ePAD parameter may be derivedfrom RV pressures at the moment of pulmonary valve opening, and heartrate may be derived from information in an intracardiac electrogram(EGM) recording.

The set of impedance electrodes 170, 172, 174 and 176 is coupled by aset of conductors 178 and is formed as a lead that is coupled to theimpedance power supply and signal processor 180. Impedance-basedmeasurements of cardiac parameters such as stroke volume are known inthe art, such as an impedance lead having plural pairs of spaced surfaceelectrodes located within the heart 10. The spaced apart electrodes canalso be disposed along impedance leads lodged in cardiac vessels, e.g.,the coronary sinus and great vein or attached to the epicardium aroundthe heart chamber. The impedance lead may be combined with thepace/sense and/or pressure sensor bearing lead.

The data stored by IMD 14 may include continuous monitoring of variousparameters, for example recording intracardiac EGM data at samplingrates as fast as 256 Hz or faster. In certain embodiments of theinvention, an IHM may alternately store summary forms of data that mayallow storage of data representing longer periods of time. In oneembodiment, hemodynamic pressure parameters may be summarized by storinga number of representative values that describe the hemodynamicparameter over a given storage interval. The mean, median, an upperpercentile, and a lower percentile are examples of representative valuesthat may be stored by an IHM to summarize data over an interval of time(e.g., the storage interval). In one embodiment of the invention, astorage interval may contain six minutes of data in a data buffer, whichmay be summarized by storing a median value, a 94th percentile value(i.e., the upper percentile), and a 6th percentile value (i.e., thelower percentile) for each hemodynamic pressure parameter beingmonitored. In this manner, the memory of the IHM may be able to provideweekly or monthly (or longer) views of the data stored. The data buffer,for example, may acquire data sampled at a 256 Hz sampling rate over a 6minute storage interval, and the data buffer may be cleared out afterthe median, upper percentile, and lower percentile values during that 6minute period are stored. It should be noted that certain parametersmeasured by the IHM may be summarized by storing fewer values, forexample storing only a mean or median value of such parameters as heartrate, activity level, and temperature, according to certain embodimentsof the invention.

Hemodynamic parameters that may be used in accordance with variousembodiments of the invention include parameters that are directlymeasured, such as RVDP and RVSP, as well as parameters that may bederived from other pressure parameters, such as estimated pulmonaryartery diastolic pressure (ePAD), rate of pressure change (dP/dt), etc.

An example of an electrogram 200 is shown in the top row of FIG. 4. Theassociated right ventricular pressure is shown in the middle row 210,while the change in right ventricular pressure over time, dP/dt 220, isshown in the bottom row and is derived from the pressure data of themiddle row. A mechanical impulse can be seen as an increase in pressureand a dP/dt maximum 222 following each electrical signal 202, indicatingthat the right ventricular electrical sensing is appropriate.

Various measurements may be used to detect the presence of mechanicalactivity. For example, pressure data may be detected and interpreted asan indication of mechanical activity of the heart. Such pressure dataincludes, for example, absolute systolic pressure, absolute diastolicpressure and pressures corrected for atmospheric pressure. Otherparameters may also be used to indicate mechanical activity. Forexample, a pressure waveform may be used to derive variables such asdP/dt, pulse pressure, estimated pulmonary arterial pressure,pre-ejection intervals, diastolic time interval and systolic timeinterval. In embodiments which monitor dP/dt or pulse pressure, noexternal reference is necessary for the pressure measurement. Inaddition, such embodiments may be less affected by sources of pressurechanges such as postural changes. Other methods of measuring mechanicalactivity of the heart which may be used include, without limitation,impedance measurements, accelerometers, and tensiometers.

The IMDs of various embodiments of the invention may store and/ortransmit sensed data using marker channels. Such marker channelsabstract information from readings such as EGMs into a more simplifiedform, a marker channel, which marks the presence of physiological eventsand the relative time of the event. For example, marker channels maymark the presence of sensed and stimulated atrial and ventriculardepolarizations. In some embodiments of the invention, pressure dataand/or other data indicative of mechanical activity may be stored and/ortransmitted by a marker channel indicative of mechanical activity. Thismechanical marker channel or mechanical sensed channel (M channel) marksthe occurrence of a mechanical event at a particular time. In someembodiments, the M channel may be a pressure-based marker channel. TheIMD may monitor pressure values such as right ventricular pressure orright ventricular dP/dt to determine that a mechanical event hasoccurred. The occurrence of the mechanical event may be noted, storedand/or transmitted using the M channel. Thus mechanical activity may becontinuously monitored while simplifying the data such that batteryusage, storage memory and data transmission are minimized.

The occurrence of a mechanical event may be determined by a measurementof mechanical activity exceeding a threshold value 230. For example, theIMD may measure RV pressure or RV dP/dt. When the RV absolute pressureor RV dP/dt exceeds the threshold 230, a mechanical event is consideredto have occurred and is marked on the M channel. For example, in someembodiments, the IMD continuously monitors RV pressure. Each time the RVdP/dt exceeds a threshold 230, such as 100 mmHg/s, it is marked on the Mchannel as a mechanical event 240 occurring at that moment. An exampleof the use of dP/dt 220 to identify mechanical activity 240 for a markerchannel is shown in FIG. 4. In this example, when dP/dt 220 crosses athreshold value 230 of 100 mmHg/s, the presence of mechanical activity240 is marked on the M channel.

In some embodiments, the threshold value 220 for detecting a mechanicalevent may be set relatively low such that all organized cardiaccontractions are interpreted and marked as mechanical events. Thethreshold 230 may be preset or may be programmable. For example, in someembodiments which monitor RV dP/dt 220, the threshold 230 for themaximum value may be anywhere from about 75 to about 125 mmHg/s. In someembodiments, the maximum positive RV dP/dt threshold 230 is set at about100 mmHg/s. The RV dP/dt threshold 230 may be set such that both weakand strong contractions are marked as mechanical activity 240. In someembodiments, the strength of the mechanical activity 240 may also bemarked, such as by using various mechanical markers to indicatedifferent levels of mechanical activity 240 on a marker channel or on adifferent channel. For example, the amplitude of mechanical activityinformation could be translated into markers indicating weak, baselineor strong activity such as M_(w), M_(b) and M_(s). Alternatively, thestrength of the mechanical activity 240 may be stored as data by theIMD. The strength of the mechanical activity 240 may be determined, forexample, by the value of dP/dt max 222, maximum pressure, or pulsepressure for each detected mechanical event 240.

In some embodiments, electrical activity markers may be stored on theirown channel or channels while mechanical data is simultaneously storedon a separate marker channel. In other embodiments, both electrical andmechanical markers may be stored on the same channel. In this way,electrical activity may be cross referenced with mechanical activity.When electrical sensing is appropriate, a mechanical event is expectedto follow each electrical event.

Embodiments of the invention may monitor the electrical and mechanicalchannels for synchrony between the electrical and mechanical events. Thepresence of an electrical event without a mechanical event may occur ina variety of circumstances such as electrical oversensing of signalsother than R-waves, such as rapid electrical rates, closely coupled PVCsthat are too rapid to produce a measurable mechanical event, pulselesselectrical activity, and electrical/mechanical dissociation. An exampleof electrogram oversensing of a signal other than an R-wave is T-waveoversensing, which occurs when a T-wave is detected and interpreted bythe IMD to be an R-wave. Such T-wave oversensing may occur, for example,during sinus tachycardia. In such a circumstance, if T-wave oversensingis occurring with every T-wave, some electrical events are true R-wavesand are associated with a mechanical event. The other electrical eventsare T-waves which are erroneously interpreted to be R-waves. TheseT-waves do not correspond to electrical stimulation of the cardiactissue and therefore have no associated mechanical event. The lack of anassociated mechanical event may therefore be used to distinguish trueR-waves from oversensed T-waves.

An example of T-wave oversensing is demonstrated in FIG. 5. In thisexample, every other detected electrical event 302 on the EG-M 300 isactually a T-wave 304 which is misinterpreted as a ventricular event andrecorded on the electrical marker channel as an electrical event 302.However, only electrical activations 302 that elicit measurableventricular contractions are recorded as mechanical events 340. Thesetrue ventricular contractions are detected in this example by the RVdP/dt 320 exceeding a threshold 330 of 100 mmHg/s. The mechanical events340 are recorded on the M Channel as strong contractions, M_(s) 350. TheIMD uses this data to calculate the mechanical interval (MR) 350 as thetime between two successive mechanical events.

The IMD may use the maximum positive dP/dt 422 and the minimum negativedP/dt 424 to determine whether an electrical event 402 measured on anEG-M 400 represents a true R-wave. An example of this is shown in FIG.6. RV pressure 400 and RV dP/dt 420 are measured simultaneously with theEG-M 400. According to some embodiments, after detection of anelectrical event 402, a window opens for measuring maximum positive andminimum negative dP/dt 422,426. An example of such an embodiment isdemonstrated in FIG. 6. The window may be, for example, in the range ofabout 400 ms to about 500 ms from the electrical event, or until thenext electrical detection occurs, whichever occurs first. During thiswindow, the maximum and minimum dP/dt 422, 424 are identified, accordingto some embodiments. As shown in FIG. 8, the IMD senses an R-wave 800and determines dP/dt within a window 802. The maximum dP/dt is thencompared to a first threshold 804. If the maximum dP/dt exceeds a firstthreshold and occurs within a window of time following the electricalevent, the electrical event is correlated with a mechanical event andthe electrical event is identified as a true R-wave 804. However, if themaximum dP/dt does not exceed the first threshold, there is nomechanical beat 808. The absolute value of the minimum dP/dt is thencompared to a second threshold 810. If the electrical event is anoversensed T-wave, there will be a negative dP/dt peak within the timewindow, as shown in FIG. 6. The negative dP/dt peak is referred to asminimum negative dP/dt 424 and indicates the presence of active andpassive relaxation of the ventricle after a contraction. Thus, if theabsolute value of maximum positive dP/dt 422 is less than a thresholdand absolute value of the minimum negative dP/dt 424 is greater than theabsolute value of a second threshold, the electrical activity isinterpreted to be a T-wave or oversensed noise 404 that is occurringfollowing a true R-wave 812. The absolute value of the second thresholdmay be, for example, about 150 mmHg/s. The absolute value of the secondthreshold may optionally be in the range of about 100 mm Hg/s to about250 mmHg/s. If both the maximum positive dP/dt and the absolute value ofthe minimum negative dP/dt do not exceed the thresholds, there has beenno contraction or relaxation and the initial test is indeterminate 814.Such an electrical event must then be interpreted to be an R-wavewithout associated mechanical activity such as might occur during rapidrhythms such as ventricular fibrillation, ventricular tachycardia,rapidly conducted supraventricular tachycardias, myopotentials, noise,mechanical alternans or a bigeminal rhythm. The presence of an R-wavesense without a mechanical event has different implications, dependingon the sensed heart rate and the sensed mechanical rate. The device maythen compare the mechanical heart rate and/or the average mechanicalamplitude to the measured electrical event rate to determine whether themeasured electrical event rate was accurate.

An example of electrical oversensing is demonstrated in FIG. 7. The toprow is a plot of the time interval between successive electrical events506 as detected by an IMD. The IMD interprets each electrical event tobe an R-wave, and thus the time interval in the first row is labeled theRR interval. The pressure data in the other rows of this figure can beused to confirm or reject the device's interpretation of the electricalevents. The second row of FIG. 7 is a corresponding plot of rightventricular dP/dt 520 and the third row is right ventricular pulsepressure 570. At the beginning of the recording, the top row reveals analternating pattern of long and short RR intervals 507, 508. The shorterinterval 508 is 250 ms in this example and reflects the interval betweenthe R-waves and T-waves (or other noise). Measurement of thecorresponding pressure 570 and dP/dt signals 520 for these short RRintervals 506 show the maximum positive dP/dt 522 less than thedetection threshold 530 of 100 mm/s and therefore there is nocorresponding pulse pressure for the oversensed T-wave. If minimumnegative dP/dt does not confirm T-wave or noise oversensing, then theIMD may consider additional information from mechanical rate, intervalor amplitude. As shown in FIG. 7, only events having a maximum positivedP/dt 522 greater than a threshold 530 are classified as mechanicalevents. The mechanical events may then be used to calculate themechanical rate or mechanical interval and the average amplitude of themechanical signal 572 (shown as pulse pressure in the example).

Referring again to FIG. 7, the RR interval abruptly decreases to lessthan 320 ms at beat number 18 500. This rate is fast enough to bedetected as ventricular fibrillation by devices which interpretventricular fibrillation to have occurred after a certain number of fastelectrical events, such as 18 electrical events. As shown in the secondrow of this figure (dP/dt 520), every other electrical beat is notassociated with a mechanical event. The average mechanical heart rate isless than 100 beats per minute (mechanical interval of 600 ms) and theaverage pulse pressure 572 (bottom row) and maximum positive dP/dt 522are within a normal range. The IMD may consider the heart rate, pulsepressure and maximum positive dP/dt to be normal if they stay within apredetermined range or above or below a predetermined threshold.Measurement of a normal, consistent mechanical rate (e.g., about 50 toabout 150 beats per minute) and a strong mechanical amplitude thereforeallow the IMD to reject the interpretation of the detected electricalevents as representing ventricular fibrillation. By monitoring bothelectrical events and mechanical events, the IMD is able to confirm thatRR intervals 506 associated with a maximum positive dP/dt 522 above athreshold 530 are true R-waves. The IMD is able to further detectoversensing of a T-wave or noise by confirming that an electrical eventis associated with a maximum negative dP/dt, the absolute value of whichis above a second threshold while a maximum positive dP/dt is notgreater than a first threshold. The IMD is thus able to confirm orreject the interpretation of an electrical signal by using mechanicaldata including measured RR intervals, mechanical intervals, andmechanical amplitudes.

By using mechanical rate and amplitude, the IMD is able to detect thepresence of T-wave oversensing and to react appropriately. Withelectrical monitoring alone and no pressure monitoring, the presence ofT-wave oversensing with short RR intervals 506, as shown in FIG. 7,could be interpreted by the IMD as ventricular tachycardia such that anelectrical shock could be inappropriately delivered for defibrillationonce the detection threshold is reached. However, by using pressuredata, the IMD is able to interpret the data to represent T-waveoversensing with adequate mechanical activity, and thus delivery of aspurious shock is avoided.

In addition to detecting a mechanical event associated with eachelectrical event, the IMD may assess the quality and rate of themechanical events in order to confirm the accuracy of the interpretationof the electrical signal and to determine the appropriate response. Forexample, as shown in the bottom row of FIG. 7, the pressure for eachmechanical event having a maximum positive dP/dt above a threshold wasused to calculate an average pulse pressure 272. In this example, theaverage pulse pressure 272 is the average of the previous 4 pressuremeasurements, including only those measurements determined to correlateto true electrical events. The average may include, for example, from 3to 8 pressure measurements. Thresholds or ranges of the magnitude of themechanical events individually, or of the averages of more than onemechanical event, may be used to determine the quality of a mechanicalevent, such as whether the mechanical amplitude is strong, weak,adequate or inadequate. The quality of the mechanical event may then bemarked on a marker channel.

The use of a mechanical activity average may be particularly useful intachyarrhythmias, where the average heart rate is high and the averagedP/dt and pulse pressure may decrease. In cases where the IMD detectedelectrogram rate and/or morphology indicates a tachycardia, the IMDcould require a threshold for mechanical rate (e.g. 60 beats/minute)and/or mechanical event amplitude (e.g. 70% of baseline pressures at60-80 beats/minute) that would ensure that the patient could maintainadequate blood pressure to maintain consciousness during the rapidrhythm. Rapid heart rhythms that are not associated with syncope (lossof consciousness) or pre-syncopal symptoms are considered to behemodynamically stable rhythms. For cases of fast, irregular rhythmssuch as atrial fibrillation, ventricular tachycardia or ventricularfibrillation, there may be true electrical activations that do notproduce a mechanical event as detected by the maximum positive dP/dtthreshold. For example, hemodynamically stable atrial fibrillation mayproduce irregular mechanical events, both in rate and amplitude. In sucha case, the mechanical rate threshold and/or the average mechanicalamplitude may be used by the IMD to determine whether a rhythm ishemodynamically stable. As shown in the example of FIG. 7, themechanical rate is approximately 100 bpm and the average pulse pressure272 remains above the pressure threshold 274 indicating that themechanical activity of the heart is adequate and no intervention isrequired. The example of FIG. 9 shows an induced episode of ventricularfibrillation. The top row shows the recorded electrogram 600 with eachvertical dashed bar denoting a sensed R-wave 602, as in an electricalventricular marker channel. The second row shows the recorded rightventricular pressure waveform 610. The third row shows the RR interval606 that is calculated by the device as the time between successiveelectrical ventricular markers 602. The fourth and fifth rows show thebeat-by-beat measurement of maximum positive dP/dt 622 and minimumnegative dP/dt 624. The sixth row is the mechanical marker 622 whichmarks a mechanical event 640 when the maximum positive dP/dt 622 exceedsa threshold 630 of 100 mm/s. In this example, the IMD detected rapid RRintervals 606 by monitoring the electrogram 600 and identified therhythm as ventricular fibrillation. The mechanical activity confirmsthat this identification is correct since no mechanical events 640 wererecorded in 2.8 seconds, equaling a rate of 21 beats per minute, whichis much slower than the minimum threshold of mechanical beat rate of 60beats per minute for ventricular fibrillation detection.

In addition to discriminating true R-waves from T-waves, the presence ofmechanical pulse alternans may be identified by some embodiments of theinvention. For example, the IMD may detect a mechanical event associatedwith each electrical event. However, the mechanical events may alternatebetween strong and weak contractions, which may be evident from themeasurement of mechanical activity. For example, mechanical eventsidentified by maximum positive RV dP/dt greater than a threshold mayoccur with each electrical event. However, the mechanical events mayalternate between a large maximum positive dP/dt or pulse pressure and asmall maximum dP/dt or pulse pressure. This alternating mechanicalresponse would have the same mechanical heart rate as electrical heartrate, but would have an alternating mechanical amplitude. Whenmechanical pulse alternans is detected, the machine may record theincident and/or may send notification to the patient and/or physician.

In some circumstances, there may be electrical signals that are notassociated with any measurable contraction 808, such as in FIG. 8. Acombination of mechanical rate and mechanical amplitude can be used bythe IMD to determine that no therapy is warranted, such as in FIG. 7.The IMD may record the incident and may send notification to the patientand/or physician.

In some circumstances, the IMD according to embodiments of thisinvention, may detect a mechanical signal with no associated electricalsignal. Such a phenomenon may occur due to undersensing, loss ofcapture, or dislodgement of a lead. In such circumstances, sensitivityadjustment or movement or replacement of the lead may be necessary.

In some embodiments, the electrical signal and the mechanical signal maybe continuously monitored, independent of each other and without the useof windowing. Thus, rather than opening a mechanical detection windowafter detection of an electrical event, the IMD may continuously monitormechanical activity, uncoupled from electrical activity. However, timewindows may be used for determining whether an electrical and amechanical signal are correlated. For example, a time intervalcorresponding to the pre-ejection interval, PEI, may be used as thewindow during which an electrical event and a mechanical event mustoccur in order to be correlated. Such windows may be monitored with eachbeat, though the timing of the window may vary depending upon heart rateand whether the beat is paced, sinus or ectopic. In some embodiments,the window may be, for example, about 400 ms or about 500 ms. In someembodiments, the window may be from a first electrical event to asubsequent electrical event, such that a mechanical event occurringbetween the first electrical event and the subsequent electrical eventsis correlated with the first electrical event. Such windows may beuseful at elevated rates, such as heart rates of less than about 400 msbetween electrical events. The correlation between an electrical andmechanical event may be evaluated by the IMD by a comparison of theelectrical events marked on the electrical channel and the mechanicalevents marked on the M channel.

In some embodiments, mechanical activity may be detected using more thanone variable related to mechanical activity. In these embodiments, theuse of more than one measurements of mechanical activity may provideconfirmation of the sensed mechanical event. For example, in someembodiments mechanical activity may be continuously sensed using rightventricular pressure 710 to derive dP/dt 720. When dP/dt 710 exceeds athreshold value 730, a mechanical contraction is noted by the IMD. TheIMD may also monitor pulse pressure 770. In some embodiments, a window780 for detection of the second measurement of mechanical activity maybe gated off of the first measurement of mechanical activity. Forexample, a window 780 for measurement of pulse pressure 770 may openwhen dP/dt 720 crosses the mechanical activity threshold 730. In thisway, pulse pressure 770 may be used to confirm that the mechanical eventsensed using dP/dt 720 was a true mechanical event and not an artifact.Thus the one or more additional variables related to mechanical activitymay serve as cross checks to ensure accurate detection andinterpretation. An example of this is shown in FIG. 10.

In addition to, or as an alternative to, detecting mechanical activityin the ventricles, some embodiments of the inventions may detectmechanical activity in an atrium. For example, the IMD may detect pulsepressure or dP/dt in the right atrium. Such embodiments may also beuseful for detection of far field R-waves, which may be associated withan over estimate of the atrial rate. As with the detection of T-waveoversensing, the IMD may detect the far field R-wave as an electricalevent but may determine that it is a far field R-wave, rather than atrue R-wave, by the lack of correlation between the far field R-wave anda mechanical event.

In some embodiments, the mechanical event data may be used by the IMD todetermine the patient's rhythm, rather than an EGM. Thus, in addition toconfirming that an electrical event detected by an EGM represents aventricular contraction, pressure data may also be used to directlymonitor cardiac activity separately from, or without, an EGM. An exampleof this is shown in FIG. 10. In this example, the mechanical rate isdetermined by the detection of right ventricular dP/dt crossing athreshold of 100 mmHg/s. Such embodiments may be useful in circumstancesin which EGM data is not available or is not reliable, such as leadfailure, lead fracture, undersensing or oversensing, andelectro-magnetic interference. For example, the IMD may detect a seriesof rapid pressure signals, determine that they are mechanical events andthat this is a tachyarrhythmia, and may initiate an appropriateresponse.

In embodiments in which the IMD is a defibrillator, blanking of themechanical activity may be appropriate following delivery of adefibrillating shock. Thus there may be a window, such as approximately160 milliseconds, during which no mechanical event recordings areobtained or recorded. This may avoid a source of artifact in themechanical event data.

1. A method of confirming that an implantable medical device (IMD) isnot oversensing electrical activity in an EGM, comprising: sensing anEGM via an implantable medical device; detecting electrical events usingthe EGM; sensing an intracardiac pressure via the IMD; determining adP/dt of the sensed intracardiac pressure; detecting the presence of amechanical event using the dP/dt, a mechanical event being deemedpresent when the dP/dt exceeds a threshold, the electrical events andmechanical events being detected independently from each other; andcomparing the electrical events to the mechanical events to identifywhich electrical events are followed by and associated with mechanicalevents, the identification of associated electrical and mechanicalevents indicating that such electrical events have not been oversensed.2. A method according to claim 1, wherein mechanical events occurringbetween a first electrical event and a subsequent electrical event aredeemed to be associated with the first electrical event.
 3. A methodaccording to claim 1, wherein the intracardiac pressure and the EGM areright ventricular and wherein the electrical events followed by andassociated with mechanical events include an R-wave.
 4. A methodaccording to claim 3, further including identifying which electricalevents are not followed by or associated with mechanical events, suchelectrical events indicating T-wave oversensing or noise.
 5. A methodaccording to claim 1, further comprising withholding defibrillation whenthe EMG indicates a tachyarrhythmia but the oversensed T-waves or noiseis indicated.
 6. A method according to claim 1, wherein the intracardiacpressure and the EGM are right atrial.
 7. A method according to claim 6,wherein the electrical events not followed by and associated withmechanical events indicate far field R wave oversensing.
 8. Acomputer-readable medium programmed with instructions for operating animplantable medical device, the medium comprising instructions forcausing a programmable processor to: receive sensed intracardiacpressure; identify mechanical events using the sensed intracardiacpressure; and store the mechanical events as a mechanical sense channel.9. A computer-readable medium of claim 8, further comprisinginstructions to ignore mechanical events occurring within a thresholdtime period after another mechanical event.
 10. A computer-readablemedium of claim 8, further comprising instructions to provide cardiacstimulation when mechanical events occur within a threshold time periodafter another mechanical event, the mechanical events indicating anarrhythmia.
 20. A computer-readable medium of claim 8, furthercomprising instructions to determine a dP/dt of the sensed intracardiacpressure, the mechanical events being identified based on the dP/dt.